The extent of necrosis of the cortical bone varied, depending on the location, but was not dependent on reconstruction with strut or mesh. The inner 50% endosteal part of the cortical wall was always necrotic. Proximally, near the defect site the cortical bone was almost completely necrotic and reduced periosteal repair of the host cortical bone was seen in most cases. At all other locations a hypertrophic periosteal reaction was observed. All goats with a femoral fracture had enormous periosteal callous repair in these sections with a large amount of fibrous ingrowth in the impacted grafts. In all reconstructions with a strut, a bony bridge from the host femur to the strut was seen, particularly in the sections from the middle level. No signs of revascularization, incorporation, or bone apposition on the struts were observed.
Microangiography showed a perfusion pattern of the impacted grafts that overlapped the area of fibrous tissue ingrowth as was observed in the hematoxylin and eosin-stained sections. Host blood vessels penetrated the graft from the edges, around the strut graft in the femurs with segmental defect reconstruction with a strut. On the edges of the defect revascularized graft was observed. Blood vessels penetrated through the open wire mesh from the soft tissue in the reconstructions with a metal mesh. A superficial zone of revascularized impacted grafts was observed. In both groups, the blood vessels did not penetrate the entire graft and the bone grafts near to the cement remained nonvascularized.
Histomorphometric examination showed that the percentage of new vital tissue ingrowth in the goats with segmental defect reconstruction with a cortical strut was smaller than the goats with segmental defect reconstruction with a metal mesh (Table 1). This difference was not significant (p = 0.13) when the percentage of revascularized graft was calculated as a percentage of the total graft. This difference was significant (p = 0.004) in the medial third, where tissue ingrowth from the host femur was minimal.
Several authors have reported a high incidence of complications with impaction grafting on the femoral side,4,5,13,18,19,23 especially when the technique is used in combination with segmental defects. Clinical results in femoral impaction grafting depend on surgical, mechanical, and histologic factors. Firm impaction of cancellous allografts should provide adequate initial stability of the implant.12,21 The presence of a segmental defect significantly decreased initial stem stability on the femoral side.3
The segmental defects were reconstructed with a strut graft or a metal mesh. In vitro, struts and meshes were equally effective in creating a stable stem reconstruction in combination with impaction grafting in the femur.3 The effect of segmental defect reconstruction on the revascularization of the impacted bone grafts under the reconstruction is unknown. Ingrowth of blood vessels and fibrous tissue from the host is required for bony healing in the long term. We hypothesized that the presence of a structural graft might decrease the ingrowth of host tissue into the impacted graft in contrast to an open wire mesh.
Incorporation of morselized bone grafts can be divided into an early phase of new bone formation and a late phase of remodeling.33 The current study focused on the early phase of incorporation in a deficient goat femur model and only one time was measured. The true long-term outcome can be determined only in a study with a longer followup. We only did reconstructions with a medial defect in the calcar region of the femur. No segmental defects on the lateral side of the femur were done in this study. Growth factors are important in the early phase of incorporation, but were not measured in this study.
One of the philosophies of bone impaction grafting includes the incorporation of impacted bone grafts in the long term. In contrast to animal studies25,27 and human retrievals on the acetabular side,11,33 long-term femoral incorporation has been reported as incomplete in humans,14,16,20,31 even in well-functioning patients.14 Under the metal mesh, the cortical wall regenerated. Beneath the cortical bone, viable bone and fibrous tissue were seen in an interface zone. Necrotic graft remnants were present near the bone cement years after surgery. Most retrievals were from older patients with a deficient femur and a mesh reconstruction, which may partially explain the incomplete incorporation.14,16,20
Our results from this study indicate that new bone formation is reduced in femurs with a segmental defect in comparison with a previous goat study with an intact femur.27 No new bone formation was observed in either group in our study. Impaction grafting in an intact femur did result in new bone formation proximally after 6 weeks followup in an in vivo goat study.27 Results from our pilot study with a followup of 12 weeks in two goats with the same defect and reconstructed with a mesh indicated that progression of the ingrowth zone in at least the mesh group can be expected. However, in contrast to 12 week-results in an intact femur, a large amount of graft resorption was observed with minimal new bone formation. Bone graft incorporation in a deficient femur with impaired vascularity seemed to be a different biologic process than impaction grafting in an intact femur. Intramedullary broaching and stripping of the femur at the outer side to allow for segmental defect reconstruction have a dramatic effect on the vascularity of the femur. Stripping of the periosteum especially seemed to have a destructive effect on the regeneration process. Results of histologic examination in human retrievals with an intact femur are unknown.
The technique used for segmental defect reconstruction had an effect on the initial host response in the first weeks after surgery. The total amount of tissue ingrowth was smaller after defect reconstruction with a cortical strut in comparison with an open wire mesh after 6 weeks followup. A decreased rate of revascularization may have an effect on the rate of incorporation of the impacted grafts in the long term. Some authors doubt whether the incorporation process of impacted bone grafts is needed for a satisfying long-term clinical outcome and claim that complete incorporation might be associated with loosening of the implants.1,14 However, we think that incorporation of impacted bone grafts is an important factor in the long-term clinical outcome.26 In our clinic, successful outcome of acetabular reconstructions28 was associated with almost complete incorporation of the bone grafts.33
Regardless of the technique used for segmental defect reconstruction, the major complication in this animal study was a femoral periprosthetic fracture in 38% of the goats. In all femurs with a periprosthetic fracture, a healing response was seen near the fracture site. The proximal reconstruction was not harmed by the fractures and the histologic observations made in the femurs with fractures were similar to the histologic results in the femurs without fractures. Therefore, we think that the presence of a periprosthetic fracture did not influence graft revascularization in the proximal part of the femur. The high number of periprosthetic fractures can be explained partly by the critical-sized segmental defect that was created, which was as much as 70% of the length of the stem. In this study, three of seven (43%) reconstructions had a femoral fracture in the strut reconstruction group. Surprisingly, goats continued walking with weightbearing even on a fractured femur. Clinically no fracture was expected in any goat, except the one goat that was euthanized after a fall and was excluded from the results. Control radiographs only can be made with goats under general anesthesia and were not made regularly. The only observation in the goats with a femoral fracture at euthanasia was a progressive limp. Although a fall was responsible for two of the five fractures in this study, the reconstructed femur offered insufficient stability for full weightbearing in both groups. The combination of a necrotic cortex after surgery and a stress peak arising from the inserted stem in combination with the segmental defect probably played a major role. Furthermore, it is known that a temporary cortical bone weakness during creeping substitution starts approximately 3 weeks after necrosis.17 Because the exact time of the fractures was unknown, this might have played an additional role. In humans, a regimen of restricted loading should be used for a longer period, which was not possible in the goats. A long-stem prosthesis to provide for a better stability in cases with large defects can be used in humans.
Segmental defect reconstruction with a cortical strut graft significantly harms revascularization of the underlying impacted grafts. We recommend using an open wire mesh for segmental defect reconstruction in combination with impaction grafting of the femur to allow for optimal revascularization in an area with impaired vascularity. However, in some situations, depending on the type and extent of the bone defect in the patient, strut grafts are preferable for mechanical reasons. Regardless of the technique used for reconstruction, a regimen of restricted weightbearing and long-stem prostheses should be used in severely defective femurs.
We thank Diny Versleijen and Leon Driessen for work on the histologic preparation.
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